Turbo with a failed seal will at times smoke intermittently until you get enough shaft play for the turbine to touch the side of the housing . You will know when this happens as it will feel like somebody lopped off a couple of cylinders. ANother drawback may be small metal particles from the compressor entering the engine.
Time for a new turbo. If you do not have a turbo timer then I suggest you get one and use it with the new turbo. Another item that will extend the life of your turbo is an accumulator that provides oil pressure after engine shutdown and before engine start up.
Hope this helps.
